Google search for G-Tech. Essentially an accelerometer that can measure your real-time HP. Jason Sands has one and mentioned he was only about 20HP lower than when measured on rollers. Could simply be because a dyno can't account for wind resistance or get a precise correction factor for the condition(s) you're in at the time. It's a small boxed unit that has a windshield mount and would allow you to see figures immediately. Just takes a little calibration using vehicle weight, RPM limit, etc. I have one myself. I bought it a few years ago and have still yet to calibrate and use it
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However, if you've been down the track and have a weight to plug in, use the calculator linked above.